ITO film treated by nitrogen plasma and the organic luminescent device using the same

ABSTRACT

Disclosed are an Indium Tm Oxide (ITO) film, wherein nitrogen-containing compounds produced by reactions of nitrogen with at least one atom selected from the group consisting of In, Sn and O atoms which are constitutional elements of ITO, or deposited nitrogen-containing compounds are present on a surface of the ITO film; and a method for preparing an ITO film, comprising the step of treating a surface of the ITO film with nitrogen plasma. An organic elect roluminescent device using the ITO film provided by the present invention as an anode shows a low voltage, a high efficiency and a long lifetime.

TECHNICAL FIELD

The present invention relates to an Indium Tin Oxide (ITO) film treatedwith nitrogen plasma, a method for preparing the same, and an organicelectroluminescent device using the same as an anode.

BACKGROUND ART

Recently, active research into an organic substance such as conjugatedconducting polymer have been made, since an organic electroluminescentdevice using poly (p-phenylene vinylene) (PPV), which is a conjugatedpolymer, was developed. Also, research into applying such an organicsubstance to a thin film transistor, a sensor, a laser, a photoelectricdevice, etc., and particularly to an organic electroluminescent device,have been progressing continuously.

Generally, an organic electroluminescent device comprises a multi-layerstructure in which thin films composed of different organic substancesare disposed between two counter-electrodes so as to increase theefficiency and the stability of the device. As shown in FIG. 1, the mosttypical multi-layer structure of an organic electroluminescent devicecomprises a hole injection layer 3 to which holes are injected from ananode 2, a hole transporting layer 4 for transporting holes, an emittinglayer 5 in which combinations of holes and electrons are accomplished,and a cathode 7. Such organic electroluminescent device may utilize thesaid layers composed of mixed materials or further comprise anadditional layer for the purpose of improving the efficiency and thelife of the device. Additionally, in order to simplify the manufactureof the device, a multi-functional material may be used to reduce thenumber of the layers contained in the device.

Meanwhile, one electrode on a substrate uses a transparent materialhaving a low absorbance to visible light so as to emanate the lightemitted from an organic electroluminescent device to outside, whereinIndium Tin Oxide (ITO) is generally used as a transparent electrodematerial and as an anode for injecting holes.

An organic electroluminescent device works according to the followingmechanism. Holes and electrons generated respectively from an anodehaving a high work function and a cathode having a low work function areinjected into an emitting layer through a hole injection layer/a holetransporting layer and an electron injection layer, thereby producingexcitons in the emitting layer. Finally, when the excitons decay, lightscorresponding to the energy concerned are emitted.

Researches into an organic electroluminescent device have been mademainly in regard to the efficiency, the life, the driving voltage andthe color of light of the device. Particularly, charge injection on theinterface between an organic electroluminescence substance and anelectrode mostly affects the efficiency and the lifetime. Therefore,intensive researches for improving the interfacial properties have beenmade.

More particularly, ITO surface treatment methods for improving theinterfacial properties between an ITO surface and a hole injection layerare known. Conventionally, ITO surface treatment methods includecleaning by ultrasonification and/or UV ozone, plasma treatments, or thelike. Among them, oxygen plasma treatments improve the efficiency andthe lifetime of, an organic electroluminescent device. See C. C. Wu etal., Applied Physics Letter, 70, 1348, 1997. It is reported that oxygenplasma-treatment on ITO surface makes the work function and sheetresistance of ITO increase and makes ITO surface more uniform. See S.Fujita et al., Japanese Journal of Applied Physics, 36, 350, 1997 and J.S. Kim et al., Journal of Applied Physics 84, 6859, 1995. Also, oxygenplasma treatments on ITO improve the hole injection by increasing a workfunction of ITO. Further, treatments with oxygen plasma can removepolluting materials present on the ITO surface, thereby improving theperformance of an organic electroluminescent device.

Additionally, the interface between an inorganic oxide, i.e., ITOelectrode and an organic hole injection layer has a relatively unstablestructure compared to a general interface between organic substances. Inorder to solve this problem, as shown in FIG. 2, a hole tunnel layer (ahole tunneling (buffer) layer) may be inserted between ITO and a holeinjection layer so as to improve the adhesion of the organic layer andthe hole injection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional organic electroluminescentdevice having a structure comprising substrate/anode/hole injectionlayer/hole transporting layer/emitting layer/electron transportinglayer/cathode.

FIG. 2 is a sectional view of a conventional organic electroluminescentdevice having a structure comprising substrate/anode/hole tunnelinglayer(buffer layer)/hole injection layer/hole transportinglayer/emitting layer/electron transporting layer/cathode, whereindrawing numeral 1 represents a substrate, 2 represents an anode, 3represents a hole injection layer, 4 represents a hole transportinglayer, 5 represents an emitting layer, 6 represents an electrontransporting layer, 7 represents a cathode and 10 represents a holetunneling layer (buffer layer).

FIG. 3 is a graph showing the brightness intensity with time in anorganic electroluminescent device according to a preferred embodiment ofthe present invention.

FIG. 4 is a graph showing the photo-efficiency with current density inan organic electroluminescent device according to a preferred embodimentof the present invention.

FIG. 5 is a graph obtained by XPS (X-ray Photoelectron Spectroscopy) ofITO surfaces treated with oxygen, argon-oxygen and nitrogen plasma.

FIG. 6 is a detailed view of a graph obtained by XPS of an ITO surfacetreated with nitrogen plasma.

DISCLOSURE OF THE INVENTION

We have found that when an ITO surface is treated with nitrogen plasmainstead of oxygen plasma, a thin film of nitrogen-containing compoundsis formed on the ITO surface, and, although the work function of the ITOsurface decrease thereby, organic electroluminescent devices using theITO anode having the thin film of nitrogen -containing compounds on ITOsurface show improved photo-efficiency, driving voltage and lifetimecompared to organic electroluminescent devices using an ITO anodetreated with oxygen plasma. The present invention is based on thisfinding.

According to an aspect of the present invention, there is provided anITO film, wherein nitrogen-containing compounds produced by reactions ofnitrogen with at least one element selected from the constitutionalelements of Indium Tin Oxide (ITO), i.e., In, Sn and O, or depositednitrogen-containing compounds are present on the surface of ITO.

According to another aspect of the present invention, there is provideda method for preparing an ITO film comprising the step for treating thesurface of the film comprising ITO with nitrogen plasma.

According to still another aspect of the present invention, there isprovided an organic electroluminescent device comprising a substrate, ananode, an emitting layer and a cathode, wherein the anode comprises theITO film according to the present invention.

The present invention will be explained in detail hereinafter.

ITO is a transparent conductive oxide and has advantages of a hightransparency, a low sheet resistance and good patterning capability. Byvirtue of these advantages, ITO is applied not only to an organicelectroluminescent device but also to an electrode material in variousfields including a liquid crystal display (LCD), a solar cell, a plasmadisplay and an e-paper. Additionally, it is applied in a technology forprotection of electromagnetic waves from a cathode-ray tube monitor andin ITO ink.

Meanwhile, ITO as an anode for an organic electroluminescent device ischaracterized by the following.

ITO is an n-type indium oxide strongly doped with Sn. Indium oxide is asemiconductor in which 2 p orbital of an oxygen ion forms a valence bandand 5 s orbital of In forms a conduction band. In general, as ITO isreduced to a certain degree, oxygen ions and doped Sn ions act asdoners. Additionally, as the concentration of these ions increases, theFermi level is located over the conduction band, and thus ITO representsmetallic properties.

The following are known: generally, ITO has a higher Sn concentration onits surface than in the inside thereof; thus, the Fermi level increasesand ITO has a low work function; when an ITO surface is treated withoxygen plasma, the surface having a high Sn concentration is etched andoxygen is supplied to the surface, thereby increasing the oxygenconcentration on the surface; therefore, oxygen plasma treatmentsincrease the work function, so that the barrier for the hole injectionmay decrease and the performance of an organic electroluminescent devicemay be improved.

However, when oxygen atoms are diffused and introduced into an organiclayer in an organic electroluminescent device, for example, into a holeinjection layer, the organic substance may be oxidized, and thus maylose the properties as a hole injection layer.

We recognized for the first time that ITO used as an anode for anorganic electroluminescent device might cause the problem of oxygendiffusion into an organic layer, and that treatments of an ITO anodewith oxygen plasma increase the oxygen concentration, and thus may causethe problem of oxygen diffusion into an organic layer. Therefore, inorder to solve this problem, an ITO film according to the presentinvention is characterized in that it comprises a surface includingnitrogen-containing compounds obtained by nitrogen plasma treatments.

When an ITO surface is treated with nitrogen plasma, some nitrogenmolecules used as a plasma discharge gas may be ionized under the plasmacondition, and react with In, Sn and O atoms present on the ITO surfaceto form a nitrogen-containing compound. Further, somenitrogen-containing compounds formed in the plasma may be deposited onthe ITO surface.

As can be seen from XPS analysis for an ITO surface treated withnitrogen plasma, a nitrogen-containing compound, for example, InN isobserved.

The nitrogen-containing compounds formed on the ITO surface can reducethe oxygen concentration on the ITO surface, and the thin film ofnitrogen-containing compound can prevent oxygen atoms from diffusinginto a hole injection layer starting from the ITO surface, therebyimproving the hole injection and the interfacial adhesion like a holetunneling (buffer) layer in an organic electroluminescent device.Therefore, the performance of an organic electroluminescent device canbe improved due to the aforesaid function of the nitrogen-containingcompounds on the ITO surface.

Additionally, nitrogen plasma reduces the Sn concentration by thesurface etching, and thus the ITO interface of an organicelectroluminescent device may be stabilized, and the lifetime and theefficiency of the device may be improved.

Nitrogen plasma used in the present invention may utilize nitrogen gas,or a mixed gas of nitrogen, oxygen, argon, hydrogen, etc., as a plasmadischarge gas. Also, ammonia gas or a mixed gas including ammonia may beutilized as a plasma discharge gas, instead of nitrogen gas.

The oxygen concentration on the ITO surface can be controlled by usinghydrogen, ammonia or oxygen in combination with nitrogen gas or ammoniagas as a plasma discharge gas. In the case of argon, surface-etchingratio is excellent. Therefore, a mixed gas containing these elements maybe utilized so as to control the uniformity of the ITO surface and theoxygen concentration on the ITO surface.

When a reactive gas, more particularly, hydrogen gas having highreactivity to oxygen is mixed with nitrogen in an amount of less than3%, or NH₃ gas is mixed with nitrogen gas, oxygen atoms present on theITO surface may be reacted with hydrogen atoms, that is, be reduced tomake the surface into an oxygen-deficiency state favorable to formnitrogen-containing compounds on the ITO surface.

Nitrogen plasma treatments are performed as the following, but are notlimited thereto.

After ITO glass is introduced into an RF plasma reactor, vacuum isapplied to a vacuum level of 1×10⁻⁶ torr by using a turbo vacuum pump,and then using a 100 sccm nitrogen Mass Flow Controller (MEC), nitrogengas flows into RF plasma reactor at 63 sccm to maintain a vacuum levelof 14 mtorr for 10 minutes. An RF output is set by using an RF generatorand an RF controller to generate nitrogen plasma. Variables for thecondition of plasma treatments include a vacuum level, an RF power and atreating time. Also, a vacuum level can be controlled by using anitrogen MFC, and an RF power and a treating time can be controlled byusing an RF controller.

The range of nitrogen plasma power that may be used in the presentinvention is preferably from 30 W to 150 W. If the RF power decreases, athin film of nitrogen-containing compound is hardly formed. On the otherhand, if the RF power is more than 150 W, ITO surface etching increases,surface uniformity decreases, the thickness of nitrogen-containingcompound layer increases, and thus hole tunneling becomes difficult.

As long as a layer formed by nitrogen plasma treatments prevents thediffusion of oxygen atoms, the concept of the present invention may beapplied to a general metal oxide conductor for an anode in addition toITO, and this is also included in the scope of the present invention.The metal oxide conductor includes, for example, Indium Zinc Oxide(IZO).

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. The following examples are illustrative only, and thescope of the present invention is not limited thereto.

Example 1

Preparation of ITO Transparent Electrode (Anode)

A glass substrate (Corning 7059 glass) coated with a thin film of ITO toa thickness of 1000 Å was introduced into distilled water in which adispersing agent is dissolved, and then was ultrasonificated. Thedispersing agent used for ultrasonication was a product available fromthe Fisher Company, and distilled water filtered two times with a filteravailable from the Millipore Company was used. After ITO was washed for30 minutes, it was repeatedly ultrasonificated with distilled water twotimes for 10 minutes. After the completion of cleaning with distilledwater, ultrasonicications of the substrate using isopropyl alcohol,acetone and methanol successively as a solvent are performed and thenthe substrate was dried.

Then, the substrate was transferred to a plasma reactor, treated withnitrogen plasma under a pressure of 14 mtorr at 50 W for 5 minutes, andthen transferred to a vacuum deposition device.

Formation of Hole Tunneling (Buffer) Layer

A compound represented by the following formula 1e was deposited on theITO transparent electrode prepared as described above by thermal vacuumdeposition to a thickness of 20 Å, thereby forming a hole tunneling(buffer) layer:

Formation of Hole Injection Layer

A compound represented by the following formula 1a,hexaazatriphenylenehexacarbonitrile, was deposited on the hole tunneling(buffer) layer by thermal vacuum deposition to a thickness of 500 Å toform a hole injection layer:

Formation of Hole Transporting Layer

A compound represented by the following formula 1c, NPB, was depositedon the hole injection layer by vacuum deposition to a thickness of 400 Åto form a hole transporting layer:

Formation of Emitting Layer

A compound represented by the following formula 1b, Alq3, was depositedon the hole transporting layer by vacuum deposition to a thickness of300 Å to form an emitting layer:

Formation of Electron Transporting Layer

A compound represented by the following formula 1d, a material for anelectron transporting layer, was deposited on the emitting layer to athickness of 200 Å to complete the formation of thin films of organicsubstances:

Formation of Cathode

Lithium fluoride (LiF) with a thickness of 12 Å and aluminum with athickness of 2500 Å were deposited successively on the electrontransporting layer to form a cathode, thereby providing an organicelectroluminescent device.

In the above-mentioned process, the deposition rate of an organicsubstance is maintained at 0.4-0.7 Å/sec. The deposition rates oflithium fluoride and aluminum used in the cathode are maintained at 0.3Å/sec and 2 Å/sec, respectively. The vacuum level is maintained at2×10⁻⁷ to 5×10⁻⁷ torr during the vacuum deposition.

Comparative Examples 1, 2, 3 and 4

Example 1 was repeated to obtain an ITO anode and an organicelectroluminescent device, except that nitrogen plasma treatment asdescribed in Example 1 was not performed in Comparative Example 1, andthat plasma using oxygen as a discharge gas, plasma using a mixed gascontaining argon and oxygen in the ratio of 2:1 as a discharge gas, andplasma using argon as a discharge gas instead of nitrogen plasma, inComparative Examples 2, 3 and 4, respectively.

Experimental Example

The photo-efficiency, driving voltage and lifetime of each organicelectroluminescent device obtained from Example 1 and ComparativeExamples 2 to 4 were compared, and the results are shown in Table 1.

The photo-efficiency is defined by measuring the brightness per unitarea (cd/m²) of an organic electroluminescent device in the presence ofan electric current with a current density of 100 mA/cm² at roomtemperature of 25° C., and dividing the obtained value by the currentdensity, and is expressed in a cd/A unit.

The driving voltage represents a voltage (V) applied between bothelectrode terminals of an organic electroluminescent device, in thepresence of an electric current with a current density of 10 mA/cm².

The lifetime represents a time at which point an organicelectroluminescent device shows 50% of the initial brightness in thepresence of an electric current with a current density of 100 mA/cm² atroom temperature of 25° C.

TABLE 1 Photo-efficiency Driving voltage (cd/A) (V) Lifetime(½) (100mA/cm²) (10 mA/cm²). (100 mA/cm²) Comp. Ex. 2 3.1 4.5  50 hours (O₂Plasma) Comp. Ex. 3 3.5 4.3 200 hours (Ar:O₂ Plasma) Comp. Ex. 4 2.9 3.7335 hours (Ar Plasma) Example 1 3.9 3.9 500 hours or more (N₂ Plasma)

As can be seen from Table 1, compared to Comparative Examples 2 and 3using oxygen plasma and mixed argon-oxygen plasma, respectively, Example1 using nitrogen plasma for treating the ITO surface shows thephoto-efficiency increased by about 25% and 10%, the driving voltagedecreased by about 15% and 10% and the lifetime increased by 1000% and250%.

Accordingly, when an nitrogen plasma-treated ITO anode is used in anorganic electroluminescent device, it is possible to provide an organicelectroluminescent device having a higher photo-efficiency, a lowervoltage and a longer lifetime compared to an organic electroluminescentdevice using an anode treated with oxygen plasma or argon-oxygen plasma.

FIG. 3 is a graph showing the lifetime of an organic electroluminescentdevice when different types of plasma are used, and FIG. 4 is a graphshowing the photo-efficiency with current density in an organicelectroluminescent device when different types of plasma are used.

FIG. 5 is an XPS graph for ITO surfaces treated with oxygen plasma,argon-oxygen plasma and nitrogen plasma. FIG. 6 is a detailed view of agraph obtained by XPS of an ITO surface treated with nitrogen plasma. Asshown in FIG. 6, two nitride peaks, which seem to correspond to InN, areobserved at the range of bond energies from 397 eV to 400 eV, and anitrite peak is observed at bond energy of 404 eV.

The work function and the atomic composition of the ITO in each ITOanode obtained from Example 1 and Comparative Examples 1 to 4 are shownin Table 2.

TABLE 2 ITO treating Work Function Atomic composition (%) condition (eV)In Sn O N Comp. Ex. 1 4.94 37.0 7.0 56.0 Non-treated Comp. Ex. 2 5.8539.9 3.0 57.1 O₂ Plasma Comp. Ex. 3 5.87 40.0 3.0 57.0 Ar & O₂ PlasmaComp. Ex. 4 5.15 40.5 2.5 56.0 Ar Plasma Example 1 4.89 35.8 2.4 48.713.3 N₂ Plasma

As can be seen from FIG. 5, FIG. 6 and Table 2, about 13.3% of nitrogenatoms are observed from the surface treated with nitrogen plasma, whichamount corresponds to about 27% of oxygen atoms.

As can be seen from Table 2, Comparative Examples 2 and 3 based onplasma using an oxygen-containing gas as a discharge gas show anincreased work function. Furthermore, as demonstrated from the resultsof XPS analysis, ITO surfaces treated with oxygen plasma have adecreased Sn concentration and an increased oxygen concentration,thereby increasing the work function.

As shown in Table 2, even if the ITO surface in Example 1 using nitrogenplasma has a decreased oxygen concentration and a decreased workfunction, it shows an increased photo-efficiency, a decreased drivingvoltage and an increased lifetime, compared to Comparative Examples 2and 3, as shown in Table 1.

Accordingly, from FIG. 5, FIG. 6 and Table 2, it is apparent that somenitrogen-containing molecules used as a plasma discharge gas are ionizedunder the plasma condition and reacted with In, Sn and O atoms presenton the ITO surface to form a nitrogen-containing compound on the ITOsurface, or are deposited on the ITO surface, and that suchnitrogen-containing compounds or nitrogen atoms may contribute toimprove the performances as described above.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, an organic electroluminescent deviceusing an ITO anode surface-treated with nitrogen plasma according to thepresent invention stabilizes the interfacial properties of a holeinjection layer or a hole tunneling (buffer) layer bynitrogen-containing compounds or nitrogen atoms formed on the surface ofthe anode, so that the lifetime and the efficiency of the device may beimproved and the device may be driven at a low voltage.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings, but, on the contrary, it isintended to cover various modifications and variations within the spiritand scope of the appended claims.

1-9. (canceled)
 10. An Indium Tin Oxide (ITO) film, whereinnitrogen-containing compounds produced by reactions of nitrogen with atleast one atom selected from the group consisting of In, Sn and O atomswhich are constitutional elements of ITO, or deposited nitrogencontaining compounds are present on a surface of the ITO film; and thenitrogen-containing compounds on the surface are formed by treating theITO film with nitrogen plasma, wherein a discharge gas comprising onegas selected from the group consisting of ammonia, a mixed gas ofnitrogen and ammonia, or a mixed gas of nitrogen and less than 3% ofhydrogen is used for the nitrogen plasma.
 11. The ITO film according toclaim 10, wherein the discharge gas used for the nitrogen plasma furthercomprises at least one gas selected from the group consisting of argonand hydrogen.
 12. A method for preparing an Indium Tin Oxide (ITO) film,comprising the step of treating a surface of the ITO film with nitrogenplasma, wherein a discharge gas comprising one gas selected from thegroup consisting of ammonia, a mixed gas of nitrogen and ammonia, or amixed gas of nitrogen and less than 3% of hydrogen is used for thenitrogen plasma.
 13. An organic electroluminescent device comprising asubstrate, an anode, an emitting layer and a cathode, wherein the anodecomprises an Indium Tin Oxide (ITO) film, and whereinnitrogen-containing compounds produced by reactions of nitrogen with atleast one atom selected from the group consisting of In, Sn and O atomswhich are constitutional elements of ITO, or deposited nitrogencontaining compounds are present on a surface of the ITO film; and thenitrogen-containing compounds on the surface are formed by treating theITO film with nitrogen plasma, wherein a discharge gas comprising onegas selected from the group consisting of ammonia, a mixed gas ofnitrogen and ammonia, or a mixed gas of nitrogen and less than 3% ofhydrogen is used for the nitrogen plasma.
 14. The organicelectroluminescent device according to claim 13, wherein the dischargegas for the nitrogen plasma further comprises at least one gas selectedfrom the group consisting of argon and hydrogen.
 15. An organicelectroluminescent device comprising a substrate, an anode, an emittinglayer and a cathode, wherein the anode comprises the ITO film obtainedby the method defined in claim
 12. 16. A metal oxide conductor for ananode of an organic electroluminescent device, wherein the metal oxideis treated with nitrogen plasma, wherein a discharge gas comprising onegas selected from the group consisting of ammonia, a mixed gas ofnitrogen and ammonia, or a mixed gas of nitrogen and less than 3% ofhydrogen is used for the nitrogen plasma.